Long-Term Ground Deformation and Instability Assessment Using Multi-Release EGMS Products: The Case of Castres, Southern France

Satellite Interferometric Synthetic Aperture Radar (InSAR) has become a key technique for monitoring ground deformation over large areas, providing spatially dense deformation time series (DefTS) that can reveal both linear and non-linear displacement behaviours. The European Ground Motion Service (EGMS) offers consistent Ortho products in vertical Up–Down and horizontal East–West directions, with multiple releases covering overlapping periods. This structure provides an opportunity to extend the temporal observation window by combining DefTS from successive releases and improving the understanding of long-term ground instability processes.

This study focuses on Castres and its surroundings in the Tarn department, southern France, using EGMS Ortho products from all four available releases covering January 2016 to December 2024. An innovative temporal merging scheme is applied to EGMS DefTS for measurement points available in multiple releases, producing ultra-long-term deformation records in both U-D and E-W components. Then, these extended DefTS are analysed to detect anomalous temporal behaviours across the full scene, with particular attention to changes in deformation dynamics, persistent deformation trends, and spatial patterns of stable, unstable, and modified ground motion.

The interpretation of detected anomalies is supported by the integration of external anthropogenic and natural factors, including precipitation history, seismic events, possible quarry blasts, groundwater-table variations, geological units, and geomorphological settings. By combining long-term InSAR-based deformation monitoring with multi-source environmental and anthropogenic information, the study aims to identify likely drivers of detected ground instabilities and deformation-pattern modifications. The work contributes to improved regional-scale geohazard assessment and demonstrates the value of temporally merged EGMS products for investigating complex, evolving ground deformation processes.